A sensor includes a physical parameter element that converts a physical parameter, such as acceleration, force, pressure, temperature, or the like into an electrically measurable signal, such as a capacitance, resistance, frequency, or charge. For example, a spring-mass-damper system may be used as a physical parameter element in an inertial sensor to generate an electrically measurable signal corresponding to an acceleration force. The sensor also includes an electronic interface, which uses the electrically measurable signal to output a voltage or digital value that is proportional to the physical parameter.
Most sensors experience a DC offset in the interface between the sensor output and the electronic components that generate the sensor circuit output signal. For example, a sensor circuit 10 is shown in FIG. 1. The sensor circuit 10 includes a sensor 14 and an electronic interface 18. The physical signal is sensed as input to the sensor 14, which generates an electrically measurable signal. A DC offset may be added to this signal before it is input to the electronic interface to generate the output signal.
In previously known sensors, the output signal of a sensor is measured for a zero value physical parameter input to identify the DC offset for the sensor. This measurement is identified during circuit construction and the DC offset value may then be stored as a data value or incorporated in a circuit element so it can be subtracted from the signal output of the sensor. The elimination of the DC offset is useful as the DC offset in some sensors may be larger than the signal that is proportional to the physical parameter being sensed by the sensor. Thus, elimination or attenuation of the DC offset enables the output range of the sensor to be used for representing the physical parameter being sensed instead of the DC offset.
Unfortunately, the DC offset does not always remain in a relatively tight range about the value measured at circuit construction. The change may arise from thermal conditioning of the sensor or changes in the wire bonds coupling the sensor to the electronic interface. A new measurement of the DC offset cannot be obtained in the field as it was during circuit construction because the input of the sensor cannot be de-coupled from the physical parameter. Consequently, the changes in the DC offset value that occur after manufacture of the sensor affect the measurement signal generated by the sensor circuit. Detection and measurement of DC offset changes during operational use of a sensor is desirable.